PROCESS MONITORING AND CONTROL

Transcription

1 Chemical Engineering Process Design PROCESS MONITORING AND CONTROL Keith Marchildon David Mody Monitoring means Measurement 1. There is no Control without Monitoring 2. Monitoring should not be done without intention of Control, in the larger sense PROCESS Automatic Control Manual control Monitoring Manual adjustment of process settings, controller set-points, and controller constants; process upgrades; trouble-shooting 1

2 MONITORING Is a key to successful on-going operation of a process Needs to be carefully planned to be useful, to be justified Often provokes a conflict with funders and with Operations May include some difficult-to-achieve measurements Includes both on-line measurements and sampling of materials Includes viewing ports Includes process performance and customer feed-back TOPICS EXAMINED IN THIS SECTION methods of measurement final control elements combinations of controls things to avoid; things to do distributed control systems. 2

3 SOME TERMINOLOGY controlled variable, CV, the quantity which it is desired kept at some value or within some range manipulated variable, MV, the quantity that will be adjusted to keep the CV at its desired value set point, SP, the desired value of CV error, the difference between CV and SP Monitoring should include all of these quantities R F A B B S Figure Ratio Control, an Example of Feed- Forward Control Most control is feed-back 3

4 L A/D D/A LIC Set point Figure Single-Input Single-Output Level Control MORE TERMINOLOGY proportional action whereby the signal sent by the controller is proportional to the amount of deviation of CV from SP reset or integral action whereby the signal increases with not only the amount but also with the time that CV deviates from the SP derivative action whereby the signal adds an extra component proportional to the speed with which CV is departing from SP. 4

5 Controlled Variable Temperature Fluid flow rate Liquid level Pressure Manipulated Variable Heating fluid pressure Heating fluid flow rate Heating fluid temperature Heating fluid valve opening Electrical current Electrical voltage Valve opening Pump speed Pressure or pressure differential Rate of in-flow or out-flow Valve opening Pump speed Valve opening Pump speed Relief flow Table Common Control Loops in Chemical Processes Other quantities that are controlled directly or by inference are compositions (via ph, conductance, refractive index, etc) E.M.F. TEMPERATURE MONITORING Figure Thermocouples, Sheathing and wells Resistance Figure Principle of the RTD 5

6 FLOW MEASUREMENT Figure Orifice Meter Figure VenturiMeter P static hole P dynamic hole Figure Pitot Tube Pressure difference, P is proportional to Velocity 2 Figure Turbine Meter 6

7 Figure Vortex-Shedding Meter EMF Figure Magnetic Flowmeter out Flow in Deflection Figure Mass Flow Meter Driving force 7

8 LEVEL MEASUREMENT Figure Liquid Level by Static Head Differential Pressure P P air or gas Figure Liquid Level by Bubble-Tube Pressure Figure Nuclear Level Gauge Figure Capacitance Level probe 8

9 Figure Ultrasonic Level detector PRESSURE MEASUREMENT Figure Bellows and Diaphragms I in Pressure Measurement Press. A Press. B Atmosphere Pressure Figure Pressure Device Configured for Differential, Gauge, and Absolute Measurement Sealed and evacuated Pressure 9

10 A C B Figure Differential Pressure Device with Balance Line FINAL CONTROL ELEMENT Usually a valve Sometimes a pump Something else, like an electrical current 10

11 Instrument air Percent of Maximum Flow Quick Open Linear Equal Perctage Percent of Rated Travel Valve Flow Characteristics: Three Types Valve capacity when fully open is described by the quantity CV, equal to (gallons/minute) / (psi pressure drop / specific gravity) for liquids 11

12 Flow control using a Centrifugal Pump at Constant Speed C restriction orifice FC FE A P B Pressure at A AC piping dead-head condition AB piping pump characteristic combined flow to C flow to B Discharge Reciprocating Pump (adjust speed or stroke length) Suction INLET Gear Pump (adjust speed) DISCHARGE 12

13 THE CONTROLLER Set point Current value of Controlled variable Controller signal Manual Over-ride Control Signal to Final Control element COMBINATIONS OF CONTROLS LIC FI Figure Level Indication & Control and Flow Indication LC FIC Figure Level Control and Flow Indication & Control 13

14 LAL LALL I LC FIC Figure Alarm and Interlock LC FIC PC TIC Heating medium Figure Cascade Control 14

15 LC FIC PC TIC Outer Loop Inner Loop PC ZC LC FIC Figure Balancing Control 15

16 LIC FI Ratio FI Figure Ratio Control Σ LC FIC Flow Set point Figure Control Using a CalculationalBlock 16

17 LC Σ TIC LE TE FIC PC Figure Multiple -Input/Multiple-Output (MIMO) Control THINGS TO AVOID Trying to control one variable simultaneously with two manipulated variables Ignoring potential interaction between two loops Creating loops that are overly tight or overly slack THINGS TO CONSIDER Inferential control; Advanced process control Mathematically simulating the process plus the controls Creating an on-line simulator to evaluate product as it is made 17

18 THE DEVELOPMENT OF PROCESS CONTROL Originally strictly manual and in the field Some process lines brought through a central point, to reduce labour required Automatic controllers invented, mounted in the field -initial versions were actuated by instrument air and incorporated mechanisms of nozzles, flapper valves and bellows Electronic controllers introduced Control signals brought into central control rooms; instruments still field-mounted Electronic controllers brought into the control room ADVENT OF COMPUTERS Direct digital control using a single big mainframe Supervisory set-point control Distributed control systems Programmable logic controllers Convergence of DDC and PLC concepts Smart individual controllers 18

19 DISTRIBUTED CONTROL SYSTEM: ESSENTIAL FEATURES AND DESIRABLE FEATURES Sufficient size to handle the number of control loops Reliability through redundancy Capability for simple and for complex control Programmability Graphic display of current operation Storage of previous information (historian) and ability to display graphically Availability of stored data for off-line analysis Alarms and intelligent alarm management Logging of events and of alarms Scheduling Recipe handling Suitable for batch or continuous 19